This invention relates to a means through which an effect of oscillations in pressurized fluid in a brake system introduced during a skid control function are dampened and thereby do not substantially effect the level of pressurized fluid requested by the operation of a control valve in brake booster during a brake application.
U.S. Pat. No. 4,514,981 discloses a hydraulic brake booster with a control valve that receives a manual input force from an operator to communicating pressurized fluid that acts on a piston and develops an output force to pressurize fluid in a control valve and effect a brake application. In this brake booster, the pressurized fluid is supplied by a pump and has a substantially constant fluid pressure. In this brake booster, a hollow stem extends from a plunger that is connected to the input rod. During a brake application, the hollow stem engages the ball on the valve member to terminate communication between an actuation chamber and a relief chamber. Once communication through the hollow stem is terminated, the ball is moved with respect to a seat and pressurized fluid is communicated to develop the output force. The pressure level of the pressurized fluid communicated to develop the output force is a substantially linear function of the input force applied to effect a brake application. Because of its simplicity and effective control of hydraulic fluid, this type control valve and adaptations thereof, have found many application in brake systems for vehicles including the brake system disclosed in U.S. patent application Ser. No. 10/159,160 filed May 30, 2002. In many vehicles having a hydraulic brake booster, additional features have been incorporated into a brake system such as skid control, traction control and sway control. These additional features are usually under the control of an Electronic Control Unit (ECU) that independently control the development of a brake application of individual wheel brakes based on a current operational condition under which the vehicle. In such brake systems, build and decay solenoid valves associated with each wheel brake are under the control of the ECU and activated to rapidly release and reapply fluid pressure to the wheel brake to maintain a safe operational condition for the vehicle. The skid control feature occurs during a brake application when the ECU determines that a wheel brake solenoid valve should be activated. During a skid control situation, oscillations may occur in the pressurized fluid supplied to the wheel brake at a frequency and duration such that the fluid pressure in the actuation chamber rises and drops rapidly. This oscillation is mirrored by the input plunger and reinforced by a return spring associated with an input plunger that urges the input plunger toward a rest position such that supply pressurized fluid is communicated to a relief chamber or reservoir even though the input force is applied to the input plunger requesting a brake application.
A primary object of the present invention is to provide a control valve for a brake booster in a vehicle with means to dampening an effect of oscillations in pressurized fluid supplied to effect a, brake application by retaining a plunger in a substantially stationary position corresponding to an input force applied to effect a brake application.
According to this invention, the control valve has first bearing member and a second bearing member located in a first axial bore of a housing. The first bearing member has a second axial bore connected to a source of pressurized hydraulic fluid. A stem located in the second axial bore has a ball retained on a first end. The ball is urged toward a seat surrounding an axial orifice in a cylindrical member that is connected to first bearing member by a first spring to define a supply chamber within the second axial bore. The supply chamber retains pressurized fluid from a source when the ball engages the seat on the cylindrical member. The axial orifice provides a flow path from the supply chamber to an actuation chamber formed between the first and second bearing members. The operational chamber is connected to the brake system that includes an ECU that can independently control the level of pressurized fluid utilized to effect a brake application of an individual wheel brake. The second bearing member has a third axial bore therein for retaining a plunger connected to an input member. The plunger has a flange on a first end located in the operational chamber and a spherical head on a second end that extends through the second bearing and into a relief chamber. The plunger has a passage that extends from the first end to a cross bore adjacent the second end while a tube extends from the first end of the plunger into the actuation chamber toward the cylindrical member. A second spring located in the operational chamber acts on the plunger to urge the flange toward and into engagement with the second bearing to establish a position of rest for the plunger. In the position of rest, the actuation chamber is in communication with the relief chamber by way of the tube, passage and cross bore. The input member includes an end cap fixed to the housing, a piston for receiving a hydraulic input and a linkage assembly for communicating an actuation force from the piston to the second end of the plunger. The actuation force acts on the second end of the plunger to initially move the tube into engagement with the ball. The engagement of the tube with the ball terminates communication from the operational chamber to the relief chamber and with further movement of the plunger, the ball is moved away from the seat to allow pressurized fluid from the supply chamber to flow into the actuation chamber for communication to the brake system to effect a brake application. The pressurized fluid in the actuation chamber may oscillate under certain operational condition caused by the introduction of skid control by the ECU. When such oscillations occur the second spring acts of the flange and may move the tube away from the ball such that pressurized fluid from the supply chamber is communicated to the relief chamber by way of said passage even though the input force remains applied to the second end of said plunger. The improvement provided by this invention resided in the plunger which is defined by a cylindrical body having a first diameter formed on the flange and located in a first diameter of the third axial bore and a second diameter that is sealingly located in a second diameter of the third axial bore. A space located between the first diameter of the plunger and the first diameter of the third bore in the second bearing member defines a dampening chamber. On initial movement of the plunger by an input force fluid is drawn into the dampening chamber and when pressurized fluid from the supply chamber is communicated into the actuation chamber additional fluid is communicated to the dampening chamber. Now should oscillations occur in the fluid pressure of the actuation chamber, fluid in the dampening chamber acts as a lock to retain the plunger in a substantially stationary position with the tube engaging the ball in accordance with the input force applied to the second end of the plunger to effect the brake application.
An advantage of this invention resides in a control valve wherein oscillation in fluid pressure caused by the introduction of a skid control feature into a brake system does not materially effect a request for the communication of pressurized fluid to effect a brake application as defined by an input force applied to a plunger.
A still further advantage of this invention resides in a relationship created between a plunger and a bearing to define a controlled hydraulic lock to maintain the plunger in a substantially stationary position corresponding to an input force for a duration of a brake application.